Abstract

Unique features of lasers, such as spatial and temporal coherence, low divergence, high power density and monochromaticity, endorse a broad spectrum of applications in fabricating nanostructured materials. Based on laser-material interactions, such as localized heating, optical near–field effect, ablation, and resonant excitation, a number of laser–assisted techniques, laser exfoliation, laser direct writing, laser–assisted chemical vapor deposition, and laser–assisted combustion synthesis, were developed in our group for fabricating carbon nanostructures of different dimensionalities, including two–dimensional graphene, one dimensional carbon nanotubes, and zero–dimensional carbon nanoonions.

Introduction

Due to its versatile bonding and atomic arrangements, the element carbon is capable of forming multiple allotropes, as shown in (Figure 1), and exhibits diverse properties [1]. The discovery of buckminsterfullerene (C60) [2] opened a magic door towards the fairyland of carbon nanostructures followed by consecutive discoveries of fullerenes, [2–6] carbon nanotubes (CNTs), [7] and graphene,[8, 9] which are then recognized as new carbon allotropes. Based on their dimensionalities, the new carbon nanostructures can be classified into three major categories, including zero–dimensional (0D) fullerenes and carbon nanoonions (CNOs), one dimensional (1D) CNTs and carbon nanoscrolls (CNSs), and two–dimensional (2D) graphene.

Figure 1: Carbon family tree showing known carbon allotropes.

Figure 1: Carbon family tree showing known carbon allotropes.

The carbon nanostructures not only provide ideal platforms for enriching fundamental understanding at nanoscale, but also inspiring ground–breaking breakthroughs in practical applications due to their diverse while remarkable properties [10–46]. The carbon nanostructures have purged several consecutive heat–waves in both scientific and engineering communities, and exerted significant impacts in almost every scientific and engineering field [10–46]. A broad spectrum of applications has been investigated to renovate traditional applications and motivate innovative products [10–46]. A number of start–up companies have emerged developing nanocarbonbased products. It is expected that the advancements of the carbon nanostructures were able to provide solutions satisfying the world’s hunger in various fields and yielding affordable products in daily life. Therefore, controlled fabrication of the carbon nanostructures is required to meet the steadily growing market.

The unique properties of lasers offer distinctive capabilities in nanofabrication [47, 48]. It has been well known that the unique lasermaterial interactions can lead to permanent changes in materials which are not easily achievable through other means [47, 48]. When projected onto material surfaces, laser irradiation can accurately deliver very low (˜ mW) to extremely high (˜ 100 kW) large power into confined regions with a precise spot size⁄dimension and spatial⁄ temporal distribution, and causes a variety of effects, including ablation, shockwave peening, photo–thermal and photo–chemical processes [47, 48]. A wide spectrum of laser wavelengths makes it possible to achieve purposeful energy delivery to a target material through resonant excitation by providing photons of matching energy to interact with the target material, such as laser–induced bond breaking and bond–selective chemical reactions [47, 48]. Easy manipulation of laser beams stimulates the development on the mask–free direct laser writing of nanostructures. Extremely intense energy delivery through laser beams results in a rapid thermal process at the laser focal point, which provides an incomparable cold–wall rapid thermal processing technique. Polarization–dependent lasermaterial interactions provide unique feasibilities in the controllable fabrication of nanostructures, such as tip–induced optical near field enhancement. As a result, a wide variety of laser–based synthetic strategies have been developed in fabricating nano–materials and structures. Since the very beginning of nano–science and technology, lasers have played a significant role in fabricating and producing desired nanomaterials and structures. For example, the laser ablation has been established as one of the most successful synthetic methodsin the scalable production of fullerenes and CNTs, [49–55] and is currently widely adopted in commercial production.

In this review article, an overview of laser–based synthetic techniques developed in the authors’ research team, Laser–Assisted Nano–Engineering (LANE) Lab at the University of Nebraska– Lincoln (UNL), for fabricating carbon nanostructures is presented,including 1) laser direct writing graphene patterns,[56, 57] 2) production of graphene via laser–assisted exfoliation of graphite,[58, 59] 3) laser–assisted controlled growth and integration of carbon nanotubes,[60–65] and 4) laser–assisted scalable production of carbon nanoonions [66].

Although the CVD method can achieve scalable growth of graphene using a roll–to–roll method, fabrication of graphene patterns, which are required developing grapheme–based functional devices, is a still a multistep time–consuming and costly process requiring expensive lithographic processes. Therefore, a convenient approach is required to achieve fast, scalable and affordable production of graphene patterns. In LANE, we developed a laser–assisted CVD (LCVD) method and successfully achieved direct writing graphene patterns using a laser as the pen and gaseous hydrocarbon precursors as the ink [56, 57]. Line–shaped graphene ribbons of controlled widths and lengths were precisely fabricated on a Ni foil without extra annealing and lithographic patterning procedures. Transparent interconnections connecting multiple electrodes were fabricated by transferring the graphene ribbons onto SiO2⁄Si substrates containing pre–deposited electrodes. The localized laser heating results in a rapid thermal process at the laser beam focal point. Graphene patterns could be fabricated at a scanning speed up to 200 μm⁄s, which is much faster and more convenient than a traditional fabrication approach. Direct writing graphene patterns using the LCVD methodexpands the capability of CVD approaches in rapid and controllable fabrication of graphene for a wide range of application.

(Figure 2) demonstrates a schematic diagram of laser direct writing graphene on a nickel (Ni) foil using the LCVD method, in which a continuous wave laser (Coherent G5, λ = 532 nm) was used as the pen and a methane–hydrogen gas mixture as the ink. The graphene growth process was performed in an environmental chamber with a base pressure of 10–3 torr. The laser beam was normally projected onto the Ni foil with a spot diameter of 20 μm. A motorized stage was used to move the Ni foil precisely following the designed pattern for writing graphene patterns. The fabricated graphene patterns can be transferred onto required substrates by using poly (methyl methacrylate) (PMMA) stamps [56, 57].

Figure 2: A schematic diagram of laser direct writing graphene.

Figure 2: A schematic diagram of laser direct writing graphene.

(Figure 3a) shows an optical micrograph of a line–shaped graphene ribbon on a Ni foil as prepared using the LCVD method. Rippled edges on the graphene ribbon were ascribed to the difference between the thermal expansion coefficients of Ni and graphene. (Figure 3b) shows a Raman spectrum of spot–A in (Figure 3a), clearly indicating the existence of monolayer graphene. Both symmetric 2D–band at 2691 cm–1 and a high 2D⁄G band ratio (around 3 – 4) confirm the existence of monolayer graphene. Invisible D–band demonstrates the high quality of the as grown graphene. (Figures 3c and 3d) show the Raman mapping of the graphene ribbon using Gband (green) and 2D–band (red), respectively. Sharp edges between the graphene ribbon and surrounding areas are observed, demonstrating precise position control in direct writing graphene. Relatively uniform Raman intensity distribution in both G– and 2D–bands indicates the homogeneity of the graphene ribbons.

The number of graphene layers could be precisely controlled by tuning the scan speed of the laser beam. (Figures 4a to 4d) show optical micrographs of the graphene ribbons fabricated on SiO2⁄ Si substrates with respect to different laser scan speeds. Graphene of different number of layers, single, double, and multi–layer, were obtained at different scanning speed. Generally speaking, the slower the laser scanning speed, the more the layer number is. (Figure 3e) shows the Raman spectra of the graphene ribbons fabricated at different laser scan speeds. (Figure 3f) shows the control of graphene layers with respect to the gas pressures and laser scan speeds.

Due to its versatile potential applications, large quantity of graphene is demanded, therefore requiring an affordable and scalable production technique. As the first successful graphene fabrication approach, mechanical exfoliation from highly ordered pyrolytic graphite (HOPG) has been demonstrated to be the most straightforward and simple method [8, 9]. However, the low throughput makes this method only applicable for lab use. The LANE developed a pulsed–laser exfoliation technique producing graphene directly from HOPG [58, 59]. By using a pulsed Nd: YAG laser (wavelength = 532 nm), exfoliation of HOPG can be performed either in vacuum or in liquid [58, 59]. Multi–layer graphene was obtained. It was found out that the formation of graphene was achieved within a laser fluence window from 1.0 to 10.0 J ⁄cm2. The formation of graphene is ascribed to the immediate compression and expansion of the HOPG, which leads to the exfoliation of graphene sheets from the layered structure of HOPG [58, 59].

Laser–Assisted Fabrication of One–Dimensional Carbon Nanotubes

Discovery of CNTs has brought a revolutionary vision to the scientists and engineers due to their unique structures and extraordinary properties [19, 24,32,46,78,79]. A wide spectrum of CNT based applications have been proposed and investigated. CNTs have been envisioned as one of the most promising candidates for fabricating next–generation electronic devices [19,24,32,46,78,79]. Extensive investigations were already made in fabricating CNTbased components and devices, such as transistors, logic gates, interconnects, sensors and detectors [13–18, 29,33, 34,38–40,43,44,80– 87]. Extensive and practical applications of CNTs in the field of electronics and devices require precisely controlled growth and integration of CNTs. Several critical topics, including where it starts, where it goes, alignment direction, stable wiring, and electrical types, have to be addressed to meet the challenges. Ideally, it is desired that CNTs of determined electrical types were precisely wired between electrodes with stable contacts for device fabrication [88–93]. However, such precise control is extremely challenging due to the ultra–tiny size of the CNTs, high–resolution nanoscale positioning, and subtle structure diversity [88–93].

To address the challenges, a series of laser–assisted techniques were developed in LANE, including optically controlled parallel integration of CNTs into pre–designed micro⁄nanoarchitectures in a single step, [60,61] selective removal of metallic CNTs through laser irradiation, [64] and growing diameter–modulated singlewalled CNTs [63]. Based on the laser assisted techniques developed, investigators in LANE successfully fabricated plasmonicenhanced CNT infrared bolometers [62].

Optically Controlled Parallel Integration of CNTs

Optically controlled parallel integration of CNTs was achieved through a LCVD method, [60,61] as shown in (Figure 6). CNTs were wired between electrodes pre–patterned on SiO2⁄Si substrates. Ultrasharp electrode tips were fabricated using focused–ion–beam (FIB) milling technique and used as optical antennas to focus and enhance optical fields within the laser irradiated regions. A continuous–wave (CW) CO2 laser (wavelength = 10.6 μm) was used to irradiate the patterned substrates. The laser beam has a polarization horizontally linearized. The SiO2⁄Si substrate was heated up to 500 °C, which is almost 200 °C lower than a conventional LCVD process, before feeding the gaseous precursors. A gas mixture of acetylene (C2H2) and anhydrous ammonia (NH3) was used as precursor for growing CNTs. (Figure 6b) shows a schematic structure of a CNT–integrated bridge structure.

Figure 6: (a) A schematic diagram of the LCVD fabrication process for growing CNTs, and (b) a schematic diagram of a CNT-integrated bridge structure.

Figure 6: (a) A schematic diagram of the LCVD fabrication process for growing CNTs, and (b) a schematic diagram of a CNT-integrated bridge structure.

(Figures 7a and 7b) show a top–view and side–view, respectively,of a typical CNT–bridge structure. Typical Raman spectrum of the CNT–bridge, (Figure 7c), shows a sharp radial breathing mode (RBM) shift at 207.6 cm–1, a Lorentzian line–shaped G– band at 1567.8 cm–1, and a strong G band at 1591.5 cm–1 indicating the existence of singlewalled semiconducting CNTs (s–CNTs) with an estimated diameter of 1.2 nm [106–108]. Simultaneous integration of CNTs into multiple sites is demonstrated in (Figure 8a, Figures 8b and 8c) exhibit zoomed images of the squared regions in (Figure 8a) and clearly show precisely integrated CNTs between each pair of tip–shaped electrodes. Only one CNT was observed between each pair of the electrodes. All CNTs are rooted at the electrode tips, indicating a site selective growth mode. The CNT–bridge structures show a nonlinear Ids-Vds curve, as shown in (Figure 8d). In the Raman spectrum, as shown in (Figure 8e), a sharp RBM Raman shift at 209.9 cm–1 and a Lorentzian line–shaped G– band are observed, and indicate the growth of s–CNTs with an estimated diameter of 1.19 nm [106–108].

Figure 8: (a) An SEM image showing an electrode pattern containing two pairs of electrodes; (b) and (c) SEM images of the squared regions in (a), showing the CNT- bridge structures; (d) I-V measurement of the device; and (e) Raman spectrum of the CNTs.

Figure 8: (a) An SEM image showing an electrode pattern containing two pairs of electrodes; (b) and (c) SEM images of the squared regions in (a), showing the CNT- bridge structures; (d) I-V measurement of the device; and (e) Raman spectrum of the CNTs.

It was found that laser polarization played a significantly role in the selective growth of CNTs. As shown in (Figure 9a to 9c), crossshaped electrodes were fabricated. It is well known that optical nearfield enhancement is a function of wavelength, material, geometry and polarization of the exciting light field [109, 110]. Under the same circumstance, the field–enhancement factor is in reverse proportion to the tip angle. The laser polarization was set to be parallel to one pair of the tips, as shown in (Figure 9a). To exclude the influence from the electrical field, no external electrical field was used. CNTs were precisely wired between tips parallel to the laser polarization as shown in (Figures 9b and 9c). Growth of CNTs showed obvious selectivity on position and orientation between the electrode tips parallel to the laser polarization. A similar nonlinear Ids-Vds curve was observed as shown in (Figure 9d). The sharp RBM peak at 204.8 cm–1, (Figure 9e), indicated the growth of CNTs with a diameter of 1.21 nm [106–108].The G– band at 1565.5 cm–1 showed a Lorentzian line shape, a characteristic of s–CNTs [106–108].

It is well known that sharp metallic tips can be used to enhance the electromagnetic field at nanoscale tip regions and result in an enhanced localized heating at the apexes of the metallic tips [109,110]. Numerical simulations, using the high frequency structure simulator (HFSS, Ansoft), were conducted. Laser beams of different polarization directions were used separately in the simulation, perpendicular (Figures 10a and 10b) and parallel (Figures 10c and 10d) to the electrode alignment. (Figures 10a and 10c) show the electrical field distribution in the metallic tips under laser irradiation of different polarization directions. Significant enhancement of the local electrical field is observed at the tips parallel to the laser polarization, almost tenfold stronger than that of the rest of the electrodes. Since a changing electromagnetic field generates an eddy current, the enhanced high–frequency E–field at the tips results in a significantly enhanced local eddy current and yields an enhanced localized heating at the nanoscale. (Figures 10b and 10d) show the heat distributions in the metallic tips under laser irradiation of different polarization directions. The local heating enhancement reaches the maximum at the tips parallel to the laser polarization, almost one order of magnitude larger than that of the rest of the electrodes. Therefore, a significant temperature increase at the tips is induced by the optical near–field effects. The simulation results confirm the hottest spots at the electrode tips parallel to the laser polarization, resulting in the selective growth of CNTs parallel to the laser polarization, (Figures 9b and 9c). Therefore, the CNT growth control is ascribed to the selective heating of electrode tips caused by the optical near–field effect, which is influenced by the laser beam polarization.

Figure 10: Electrical field distribution (a) and heat distribution (b) within the electrodes with the laser beam polarization normal to the electrode tip pair; electrical field distribution (c) and heat distribution (d) within the electrodes with the laser beam polarization parallel to the electrode tip pair.

Figure 10: Electrical field distribution (a) and heat distribution (b) within the electrodes with the laser beam polarization normal to the electrode tip pair; electrical field distribution (c) and heat distribution (d) within the electrodes with the laser beam polarization parallel to the electrode tip pair.

Selective Removal of Metallic Cnts through Laser Irradiation

LANE achieved selective removal of metallic CNTs (m–CNTs) through a post–growth laser processing [64]. An optical parametric oscillator (OPO) wavelength tunable laser (Continuum, Panther ® EX) was used as the laser irradiation source, which provides laser emission ranging from 215 to 2550 nm. Pulsed laser beams with a wavelength of 2 μm (power density of 1.8 MW⁄mm2.) were used for the selective removal process. Laser beams of shorter wavelengths could cause obvious damage to both s– and m–CNTs due to the highenergy photons. Wavelengths larger than 2 μm were not investigated due to the sharp drop of the output power for wavelength over 2 μm from the OPO laser.

Well aligned CNT arrays between parallel electrodes were fabricated, resembling a function of CNT–based field–effect transistors (CNT–FETs). It is well known that m–CNTs and s–CNTs show distinctly different electrical transport behaviours. CNT arrays containing m–CNTs exhibit linear electric transport features independent of Vg. (Figures 11a and 11b) show the Id-Vg and Id- Vds curves of a CNT array before laser irradiation with obvious existence of m–CNTs confirmed by the linear curves and absence of Vg dependence. The CNT array was irradiated using the OPO laser. After the laser irradiation, (Figures 11c and 11d), I-V curves show nonlinear semiconducting behavior and obvious Vg dependence. SEM images of the same sample were taken before and after the laser irradiation to compare the physical changes of the CNTs, as shown in (Figure 12). All CNTs were labeled before laser irradiation. Afterlaser irradiation, CNTs 3 and 2 are removed in (Figures 12b and 12d), respectively.

Figure 12: SEM images of the CNT arrays before, (a) and (c), and after, (b) and (d), laser irradiation. Each number marks to the same CNT before and after laser irradiation.

Figure 12: SEM images of the CNT arrays before, (a) and (c), and after, (b) and (d), laser irradiation. Each number marks to the same CNT before and after laser irradiation.

Raman spectroscopic investigations were carried out to verify the selective removal of m–CNTs. (Figure 13e) shows the Raman spectra from three different regions on a CNT sample, as shown in (Figures 13a to 13d). In the laser irradiated spot, (Figure 13d), the G––band shows Lorentzian line shape only, indicating the existence of s–CNTs only in this region [111–113]. In the region away from the laser irradiation spot, (Figures 13b and 13c), the G––band turns to showing a BWF line shape, indicating the existence of m–CNTs.[111–113] The insets in (Figure 13e) show zoomed views of the RBM signals from different areas of the sample. RBM signals coming from m– CNTs (in the rectangular boxes) were observed in the regions which were not irradiated by the laser beams. Only RBM peaks arising from s–CNTs were observed within the laser irradiated region, indicating the selective removal of m–CNTs.

Figure 13: (a) An optical image showing an electrode with a circle indicating the OPO laser spot. (b) - (d) SEM images collected at the indicated regions. (e) Raman spectra obtained from the selected regions corresponding to the SEM images; the insets are the close-up views of the RBM peaks.

Figure 13: (a) An optical image showing an electrode with a circle indicating the OPO laser spot. (b) - (d) SEM images collected at the indicated regions. (e) Raman spectra obtained from the selected regions corresponding to the SEM images; the insets are the close-up views of the RBM peaks.

Selective removal of m–CNTs is ascribed to the electronic properties of the m–CNTs due to (1) higher infrared absorption rate than s–CNTs, [114] (2) optical near–field effect on m–CNTs, and (3) free electron movement in m–CNTs. It was reported that m–CNTs exhibited a much larger absorption rate at infrared range than s–CNTs due to Drude like absorption [114]. Under the laser irradiation, m–CNTs act like nanoantennas and focus the incident infrared irradiation at the local points and enhance the optical absorption. The increased infrared absorption will significantly increase the temperature of the m–CNTs selectively. At the same time, free electrons in conduction bands of m–CNTs respond readilyto the electromagnetic field generated from the incident irradiation. Therefore, m–CNTs are more responsive to the incident infrared irradiation than s–CNTs, and selectively oxidized when exposed to air.

Growing Diameter–Modulated Single–Walled CNTs

The band gaps of a semiconducting single–walled CNT are inversely proportional to its diameters [115]. Since the growth of single–walled CNTs is sensitive to the environmental temperature, [116, 117] a controlled periodic temperature change in the growth process is expect to obtain a periodic change in the diameters of singlewalled CNTs, [116, 117] implying the possibility to achieve band gap engineering within individual single–walled CNTs for the fabrication of functional devices. New electronic and optical properties can be expected from the diameter–modulated single–walled CNTs, such as improving optical absorption in solar cells by absorbing different wavelength regions in the solar spectrum in different sections of the CNTs.

LANE achieved facial synthesis diameter–modulated CNTs by controlling substrate temperatures instantly through tuning the incident laser power [63]. Single–walled CNTs with increasing diameters were grown by reducing the growth temperature from 650 to 450 oC, as shown in (Figure 14). In Raman spectra of the vertically aligned single–walled CNTs, both low and high frequency RBM peaks were observed at the starting end of the CNT, while only low frequency RBM peak was observed at the other end, indicating the increase in tube diameters from one end to the other.

Figure 14: A TEM micrograph of a single-walled CNT grown with a descending temperature profile from 650 to 450 oC. The zoomed views, (1 - 5), corresponding to the labeled areas of the tube, show the tube diameter changes from one end to the other.

Figure 14: A TEM micrograph of a single-walled CNT grown with a descending temperature profile from 650 to 450 oC. The zoomed views, (1 - 5), corresponding to the labeled areas of the tube, show the tube diameter changes from one end to the other.

I–V characteristics and Raman spectra of CNT–FETs fabricated with a diameter–modulated and a regular single–walled CNT are compared and shown in (Figure 15 and 16). The device with the diameter–modulated single–walled CNT exhibits a diode–like I–V curve, Figure 15, whereas the device with the uniform one has an almost symmetrical I–V curve, (Figure 16). The device with the diameter–modulated single–walled CNT forms two different contacts with the metallic electrodes. One side forms a Schottky barrier between the Mo electrode and the single–walled CNT end with the smaller diameter (a larger band gap). The other side formsan Ohmic contact between the Mo electrode and the single–walled CNT end with the larger diameter (a smaller band gap). The band diagram of the device is shown in the inset of (Figure 15d). When the device was positively biased (Vds > 0) and the Schottky contact (source) was grounded, positive current value was obtained. When negatively biased (Vds < 0), no current flew through the device. Based on the previous studies, single–walled CNTs can form either Schottky barriers or Ohmic contacts with different metals, depending on the work function of the metal to contact with. Using a uniform single–walled CNT, forming a diode by contacting the tube with two different metals would increase the fabrication complexity, time, and cost [118–120]. Using a diameter–modulated single–walled CNT, different band gaps at the two ends of the tube can form a diode by simply bridging the single–walled CNT with two electrodes of the same metal. This approach significantly simplifies the processes to fabricate CNT–based Schottky diodes. Micro–Raman spectroscopy of the single–walled CNTs was conducted by mapping the tubes with 1 μm steps to investigate the diameter modulation and corresponding band gap changing along the tubes as assigned in the Kataura plot [121]. As shown in (Figure 15), the RBM peak frequency is much higher on one side (grown under higher temperature) and shifts to a lower frequency on the other side (grown under lower temperature) of the single–walled CNT. The double–peak feature obtained at position2 indicates an inter–junction area since the higher peak is the same as that of position 1 and lower peak is the same as that of position 3 of the tube. At the same time, it was observed that the electrical currents were much smaller in diameter–modulated single–walled CNTs than regular tubes, due to the electron scattering caused by the defects.

Figure 15: (a) SEM image of a single-walled CNT bridging Mo electrodes. (b) Raman spectra at different locations on the tube shown in (a). (c) The schematic representation of the tube structure. (d) The typical I-V characteristics showing a diodelike behavior of the device with an inset indicating the expected band diagram.

Figure 15: (a) SEM image of a single-walled CNT bridging Mo electrodes. (b) Raman spectra at different locations on the tube shown in (a). (c) The schematic representation of the tube structure. (d) The typical I-V characteristics showing a diodelike behavior of the device with an inset indicating the expected band diagram.

Figure 16: (a) SEM image of a single-walled CNT bridging the Mo electrodes. (b) The corresponding Raman spectra. (c) The schematic representation of the structure. (d) I-V characteristics showing almost symmetrical conductance in both directions with an inset indicating the expected band diagram.

Figure 16: (a) SEM image of a single-walled CNT bridging the Mo electrodes. (b) The corresponding Raman spectra. (c) The schematic representation of the structure. (d) I-V characteristics showing almost symmetrical conductance in both directions with an inset indicating the expected band diagram.

Fabricating Plasmonic–Enhanced CNT Infrared Bolometers

Based on the laser–assisted CNT growth and integration techniques, LANE successfully fabricated plasmonic–enhanced CNT infrared (IR) bolometers by integrating CNTs into metallic nanoantenna arrays, [62] as illustrated in (Figure 17). Due to the strong localized optical field in the gaps of the nanoantenna arrays, a responsively of 800 V⁄W was achieved at room temperature [62].

Laser–Assisted Fabrication of Zero–Dimensional Carbon Nanoonions

Carbon nano–onions (CNOs) are concentric multilayer giant fullerenes, which consist of multiple concentric graphitic shells to form encapsulated structures [122]. As an important member in the fullerene family, CNOs have been investigated extensively to replace fullerenes due to their extreme similarities shared with fullerenes but at a much lower cost [4, 123–128]. A wide spectrum of applications have been envisioned for CNOs, including gas storage, super capacitors, broadband electromagnetic shielding, catalyst support structures, nanolubricating additives, and water purification [4, 123– 133]. Therefore, scalable production of quality CNOs is desired.

In LANE, a laser–assisted combustion method, (Figure 18), was developed to achieve catalyst–free and scalable growth of CNOs in open air by using laser irradiations to achieve resonant excitation of the precursor molecules - ethylene [66]. Both growth rate and crystalline quality of the CNOs were significantly improved by matching the laser irradiation wavelengths with the vibrational states of the ethylene molecules. Highly concentric CNO particles were obtained at a laser wavelength of 10.532 μm, which resonantly excited the CH2 wagging mode (ν7, 949.3 cm– 1) of the ethylene molecules. A growth rate up to 2.1 g h–1 was recorded, which is higher than that of without laser (1.3 g h–1).

The ethylene–oxygen flame becomes obviously brighter and shorter (Figure 19c) when resonantly excited (10.532 μm) compared with the cases without laser excitation (Figure 19a) and a nonresonant excitation at 10.333 μm (Figure 19b). CNOs produced under the resonant excitation conditions (10.532 μm) demonstrate obviously improved crystallinity showing concentric structures containing long–range ordered graphitic striations. (Figure 19g) shows typical Raman spectra of the CNOs. The inset shows a zoomed view of the second–order Raman peaks. In the Raman spectra, the Dand G–bands are observed at 1350 and 1590 cm–1, respectively, and fitted into five different bands (G, D1, D2, D3, and D4), [134, 135] as shown in (Figure. 19h) Herein, the full width at half maximum (FWHM) of the G–bands and the relative intensities (R3= ID3⁄(ID3 + ID2 +IG)) of the D3 bands were analyzed [134, 135]. By comparing the CNOs grown under different circumstances, the decreased G–band FWHM, reduced R3 ratio, and increased 2D–band ((˜ 2690 cm–1) intensity indicates the improvement in the crystallinity of theCNOs grown with the resonant excitation at 10.532 μm. The CNO growth rate at 10.532 μm was measured to be around 2.1 g h–1, which was higher than that at 10.333 μm (0.3 g h–1) and without laser (1.3 g h– 1). Therefore, the resonant excitation of the C2H4 molecules at 10.532 μm not only improves the crystallinity of CNOs but also increases the growth rate.

Figure 19: Photographs of ethylene-oxygen flames: (a) without laser excitation, (b) excited at 10.333 μm, and (c) excited at 10.532 μm (The insets show molecular vibration under the excitation conditions). TEM images of CNOs grown (d) without laser excitation and with laser excitations at (e) 10.333 and (f) 10.532 μm. (g) Raman spectra of CNOs grown without laser excitation and with laser excitations at 10.333 and 10.532 μm (The inset shows the magnified view of the second-order Raman spectra). (h) Typical curve fitting of a firstorder Raman spectrum.

Figure 19: Photographs of ethylene-oxygen flames: (a) without laser excitation, (b) excited at 10.333 μm, and (c) excited at 10.532 μm (The insets show molecular vibration under the excitation conditions). TEM images of CNOs grown (d) without laser excitation and with laser excitations at (e) 10.333 and (f) 10.532 μm. (g) Raman spectra of CNOs grown without laser excitation and with laser excitations at 10.333 and 10.532 μm (The inset shows the magnified view of the second-order Raman spectra). (h) Typical curve fitting of a firstorder Raman spectrum.

To study the influence of laser–induced resonant excitation of ethylene molecules on the CNO growth, laser absorption by the flame at the non–resonant wavelength of 10.333 μm and the resonant wavelength of 10.532 μm was investigated, as shown in (Figure 20). A much stronger absorption was observed at the wavelength of 10.532 μm, and ascribed to the resonant excitation of the CH2 wagging mode (ν7, 949.3 cm–1) of the C2H4 molecules. By absorbing laser energy at 10.532 μm, the CH2 wagging mode is resonantly excited. In this approach, laser energy was coupled into the flame more efficiently. The total absorbed laser power is much higher at an excitation laser wavelength of 10.532 μm than a non–resonant one, such as 10.333 μm, as shown in ( 20). At the laser wavelength of 10.532 μm, starting from the threshold laser power of 400 W, sufficiently high temperature in the flame is reached by the efficient coupling of the laser energy through resonant excitation to decompose polycyclic aromatic hydrocarbons (PAHs) into C2 species, which favor the formation of more ordered graphitic layers [136]. Hence, CNOs with long–range ordered concentric graphitic shells are formed.

Figure 20: The laser powers absorbed by the combustion flame at the wavelengths of 10.333 and 10.532 μm as the functions of the incident laser power.

Figure 20: The laser powers absorbed by the combustion flame at the wavelengths of 10.333 and 10.532 μm as the functions of the incident laser power.

Conclusion

A series of laser–assisted nano–fabrication techniques have been developed in LANE for fabricating various carbon nanostructures and related devices by making use of unique laser material interactions. The unique capabilities of lasers provide irreplaceable approaches addressing grand challenges in the nanostructure fabrication and integration. It is expected that lasers would find more applications in future nanostructure fabrication and device manufacture.

Acknowledgements

The author would like to express his sincere gratitude to his colleagues in the Laser–Assisted Nano Engineering (LANE) laboratory, http:⁄⁄lane.unl.edu, in the Department of Electrical Engineering at the University of Nebraska–Lincoln. Led by Prof. Yong Feng Lu, the LANE carries out vibrant research in laser–related nanoscience and technology aimed at understanding laser–material interactions at the nanoscale and molecular levels and developing application–oriented laser techniques.

The author is also heartily grateful to National Science Foundation (ECCS 0652905, ECCS 0621899, CMMI 0758199, CMMI 0852729, and CMMI 0555884),. Office of Naval Research (MURI N00014–05– 1–0432 and N00014–09–1–0943), Nebraska Research Initiative, and Nebraska Center for Energy Science for their financial support.